This invention relates to an apparatus and a method for position detection of high accuracy, to be used, for example, in relative alignment between a photomask and a wafer in the manufacture of semiconductor circuit devices, or in absolute alignment of a wafer in a wafer inspecting device.
The relative alignment is disclosed in U.S. Pat. No. 4,167,677, U.S. Pat. No. 4,199,219 or the like. According to these disclosures, a photomask and a wafer, for the manufacture of semiconductor circuits, are provided with alignment mark patterns, respectively, the photomask and wafer being disposed with a minute spacing or a projection optical system intervened therebetween. The mark patterns of the photomask and the wafer are scanned by a laser beam having spot-like or bar-like cross-section to detect the relative position of the mark patterns to thereby measure the degree of misalignment between the mask and the wafer.
FIG. 1 shows the relation between an automatic alignment mark pattern (which will hereinafter be referred to simply as "AA pattern") formed on a wafer and an automatic alignment signal (which will hereinafter be referred to simply as "AA signal") obtainable therefrom by the laser beam scanning and photoelectric conversion. In FIG. 1, a part (A) shows in an enlarged cross-sectional view the AA pattern having edges 1 and 2. When the wafer surface is scanned by a laser beam in the direction parallel to the sheet of the drawing, the laser beam is diffracted and scattered by the edges 1 and 2 of the AA pattern. The diffracted and scattered light beams are received by photodetecting means, not shown in this Figure, so that they are converted into an electric signal having a waveform such as shown at a part (B) of FIG. 1. Pulses 1a and 2a of the waveform correspond to the edges 1 and 2 of the AA pattern. These pulses are cut at a predetermined threshold V and, by binarization, they are reformed into pulses 1b and 2b shown at a part (C) of FIG. 1. The thus reformed signal is compared with a similar signal obtained in respect to a mask (not shown) or is compared with a reference established in the photodetecting means, whereby an error signal for achieving the alignment is produced.
After completion of the alignment, an exposure step is effected wherein the mask is illuminated by exposure light so that a photoresist material applied onto the wafer is exposed to an actual element pattern (circuit pattern) of the mask.
As is well known in the art, the photoresist layer of transparent material has already been applied onto the wafer prior to the above-described position detection of the wafer. It has been found that the presence of such transparent material layer substantially influences the detection of the AA pattern through the transparent material layer. More specifically, it has been found that, when the wafer is coated with a photoresist layer, the width of the pulse corresponding to the edge of the AA pattern becomes greater than that obtainable from the same edge of the AA pattern when no photoresist layer is formed on the wafer. This phenomenon degrades the signal detecting accuracy.
FIG. 2 illustrates the principle of such phenomenon. In FIG. 2, a part (A) shows the edge 1 of the AA pattern formed by a concavity defined in the wafer surface. The wafer surface is coated with a photoresist layer 3 so that, according to the direction of inclination of the stepped portion 1 of the AA pattern, the surface of the photoresist 3 is inclined downwardly in the rightward direction as viewed in FIG. 2. According to the known dark-field laser scanning technique, such as disclosed in the above-described U.S. Patents, the scanning laser beam is perpendicularly incident on the wafer surface. If the portion of the wafer surface on which the beam is incident comprises a mirror surface, the incident light is specularly reflected so that it goes back along the oncoming path such as denoted by the hatched region 4. If on the other hand, the laser beam is incident on the edge of the AA pattern, the laser beam is scattered and diffracted by the edge so that the diffracted rays trace regions 5 and 6 as well as the region 4. At a pupil plane of the alignment optical system (alignment scope), the reflected light from the wafer is subjected to the spatial frequency filtering whereby the direct reflection light, tracing the region 4, which has been specularly reflected from the wafer (i.e. the reflected light which has not diffracted or scattered by the edge of the AA pattern) is intercepted; whereas the scattered and diffracted light tracing the regions 5 and 6 is transmitted so that it is directed to a photoelectric detector (not shown in this drawing). In this manner, the known dark-field scanning technique selects only the scattered light and directs it to the detector to obtain an AA signal.
Now, the influence of the presence of the photoresist coating on the detection of the edge of the AA pattern will be considered. In the part (A) of FIG. 2, the laser beams 7 and 8 are not directed to the edge of the AA pattern. Since, however, the surface of the photoresist layer is inclined as described in the foregoing, the laser beams 7 and 8 are deflected by the prism action at the resist surface so that the reflected rays of the laser beams 7 and 8 will travel in a region such as 5 or 6 along which the scattered light from the edge of the AA pattern travels. Since the reflected rays of the laser beams 7 and 8 tracing the region 5 or 6 will also be detected by the photodetector, the resulting AA signal contains a pulse of greater width such as shown in a part (C) of FIG. 2 as compared with the AA signal shown in a part (B) of FIG. 2 which is obtainable from the same edge 1 when no resist layer is formed on the wafer surface. As will be easily understood, the increase in the pulse width varies depending on the inclination or the like of the resist coating. The variation of the pulse width of the signal corresponding to each edge of the AA patterns is a serious factor of error in the position detection of the edge.
In order to avoid this, it may be contemplated that the resist material on the part of the AA pattern is removed. However, this requires an additional step for removing the resist on the AA pattern.